There are still some people, including me, playing Unreal Tournament 99 with community patch 469.
I believe there are also some people playing UT 2004.
There are projects like UT4 Unofficial Update and Open Tournament that are focused around UT4 community modding/development. Still need to look into those.
Xonotic is a free alternative. Fun from what I've played.
https://xonotic.org/
Diabotical was pretty fun but I believe only a handful of players are still playing it.
Quake Champions could be an option, still a few hundred people playing.
The communities for these games seems pretty small right now, but I believe it's only because the games are so old. I wish we'd see some kind of UT revival. UT3 was supposed to be re-released as UT3X, but that never happened. There was definitely hype for it. I think a new game, done properly, has big potential.